Chroma quantization parameter extension

ABSTRACT

The quantization parameters (QP) for Chroma are extended up to and more preferably to the same range as Luma QP (e.g., 0 to 51). Previous, values of Chroma QP only extended up to 39. Techniques are provided for determining extended Chroma QP values (e.g., for Cr and Cb) based on the Luma QP and picture level chroma offsets. In one preferred embodiment, slice level offsets are added making the method particularly well-suited for slice level parallel processing. The extension of Chroma QP enhances functionality, flexibility and friendliness of the High Efficiency Video Coding (HEVC) standard for various applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 13/744,759, filed Jan. 18, 2013, and claims thepriority from U.S. provisional patent application Ser. No. 61/589,191filed on Jan. 20, 2012, U.S. provisional patent application Ser. No.61/623,884 filed on Apr. 13, 2012, and of U.S. provisional patentapplication Ser. No. 61/624,870 filed on Apr. 16, 2012, the entirecontent of which is hereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

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NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. §1.14.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention pertains generally to video encoding and decoding, andmore particularly to determination of a chroma quantization parameterwithin video encoders and decoders.

2. Description of Related Art

The communication and storage of videos in an efficient manner requirescoding mechanisms for reducing spatial and temporal redundancies.Although a number of coding techniques exist, ongoing efforts aredirected at increasing efficiencies of these enCOder/DECoders (codecs)which respectively compress and decompress video data streams. Thepurpose of codecs is to reduce the size of digital video frames in orderto speed up transmission and save storage space. Video coding advancesmade over the years have collectively contributed to the high levels ofcoding efficiency provided by state-of-the-art codecs. It is desired,however, that coding be performed at still higher efficiencies tofurther decrease video bit rates.

The latest of these developing coding standards is referred to as HighEfficiency Video Coding (HEVC), from the Joint Collaborative Team onVideo Coding (JCT-VC), which is a joint effort of the MPEG and VCEGstandardization committees. HEVC employs Coding Unit (CU) structure,whose main difference from a macroblock structure (e.g., in previousMPEG-2 or AVC codecs) is that instead of a fixed size (e.g., 16×16), thesize can vary up to 128×128. One Coding Tree Unit (CTU) represents bothflat area and busy area, whereby providing a single QP value for one CTUis insufficient for obtaining high levels of subjective quality.Accordingly, HEVC partitions the CTU into Coding-Units (CU), each ofwhich are represented by their own QPs which can differ from one CU toanother.

The current Chroma QP derivation process in HEVC (e.g., HM 5.0)replicates that of the H.264/AVC specification as shown in Table 1. ForQP values in the range of 0 to 29 a linear relationship (QP_(C)=QP_(Y))is followed, whereas a nonlinear relationship is followed for higher QPvalues. Chroma QP also saturates at a maximum value of 39 without anyconsideration of the color format that may be used. It will be notedthat this table actually defines the relationship between Luma andChroma at different quality levels.

However, limiting Chroma QP in the range of [0, 39] has severaldisadvantages.

BRIEF SUMMARY OF THE INVENTION

In the high efficiency video coding (HEVC) standard test model HM 5.0,Chroma QP can only take values in the range of [0, 39]. The presentinvention extends Chroma QP, up to and including the range of [0, 51] toenhance functionality, flexibility and friendliness of the HEVC codingstandard for various potential applications. These applications includerate control at low bit-rate, while providing the flexibility to obtaina desired performance balance point between luma and chroma, and forhandling video sources with different color formats (e.g., RGB).

Further aspects of the invention will be brought out in the followingportions of the specification, wherein the detailed description is forthe purpose of fully disclosing preferred embodiments of the inventionwithout placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a block diagram of extended chroma quantization parameter(ECQP) use in a video encoder according to an embodiment of the presentinvention.

FIG. 2 is a block diagram of extended chroma quantization parameter(ECQP) use in a video decoder according to an embodiment of the presentinvention.

FIG. 3 is a graph of current HEVC HM 5.0 mapping of chroma QP having alower range than luma QP utilizing the g_aucChromaScale[52] table.

FIG. 4 is a flowchart of a general process for extending the chromaquantization parameter according to at least one embodiment of thepresent invention.

FIG. 5 is a flowchart of a slice level process for extending the chromaquantization parameter according to at least one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The current HEVC standard (HM 5.0) limits QP values in a linear range upto 29 and a nonlinear range up to 39. There are a number of problemswhich arise from limiting QP in this manner.

Firstly, problems in the bit consumption for luma and chroma arise whenthe bit-rate is very low. Experimental results using HM 5.0 show that athigh QPs (e.g., for QP equal to 38, 42, 46, 50) luma and chroma consumealmost the same number of bits for their residual coding, which issignificantly different from what is observed for the common testconditions (i.e., for QP equal to 22, 27, 32 and 37). Under common testconditions, the ratio of luma residual bits and chroma residual bits isaround 9:1. With the limitation of chroma QP, rate control algorithmsmay have difficulty in allocating or finding the desired bit budgetbalancing point between luma and chroma. In the scenario of low bit-rateapplications, rate control algorithms may need to limit chroma bits byincreasing chroma QP, while saturating at 39 for chroma QP could changesuch attempts to unachievable tasks. A second problem is that thisproperty does not allow the codec to properly quantize chroma to thedesirable extent.

Another problem for rate control is that the previous observation makesbit-rate much more unpredictable and possibly unstable. In general, forrate control algorithms, a model is needed for bit to QP mapping. Quitecommonly, algorithms tend to use luma information (distortion) andignore chroma. As formerly mentioned, chroma bits at low bit-rates canhave a severe impact in the total bit-rate. Additionally, the modelsused quite commonly follow a quadratic relationship between bitrate andQP. Given the nonlinear behavior of chroma QP that model may be lessthan accurate.

Video sequences may have color components that might not follow thepossibly nonlinear relationship between luma and chroma and their impacton subjective quality that seems to have inspired the chroma QPderivation process. Instead, the present invention provides moreflexibility in controlling these QP parameters, handling a more genericcontent space, while also being able to properly handle other colorspaces and formats (e.g. RGB or YCoCg among others). Video codingstandards should provide the functionality and flexibility for videosources with different color components or even other color formats(e.g., RGB and YCoCg). When video sources have a different relationshipbetween Luma and Chroma components, assumed by Table 1, the codec maybehave unpredictably.

The present invention provides an extended chroma quantization parameter(QP) by replacing or altering the mapping table for determining thechroma QP.

FIG. 1 through FIG. 2 illustrate example embodiments of a codingapparatus comprising an encoder 10 and decoder 50 configured accordingto the invention for coding using an extended chroma QP mechanism of thepresent invention.

The encoder 10 shown in FIG. 1 has encoding elements 12 executed by oneor more processors 46. In the example, video frame input 14 is shownalong with reference frames 16 and frame output 18. Inter prediction 20is depicted with motion estimation (ME) 22, and motion compensation (MC)24. Intra prediction 26 is shown with switching between inter and intraprediction. A sum junction 28 is shown with output to a forwardtransform 30, quantization stage 32, extended chroma QP determination34, and CABAC coding 36. An inverse quantization 38 is performed alsousing the extended chroma QP determination 34, followed by inversetransform 40 shown coupled to a summing junction 42 and followed by afilter 44, such as a deblocking and/or Sample Adaptive Offset (SAO). Forthe sake of simplicity of illustration, the extended chroma QPdetermination 34 is shown being utilized during quantization stages,however, it should be appreciated that the extended chroma QP can beutilized by other blocks within the encoder and/or decoder withoutlimitation, such as within the deblocking filter, and while making modedecisions and performing motion estimation.

It should be appreciated that the encoder is shown implemented with acomputer processing means 46, such as comprising at least one processingdevice 48 (central processing unit (CPU), microcontroller, applicationspecific integrated circuit (ASIC) containing a computer processor,parallel processing devices, or other devices configured for executingprogrammed instructions) and at least one memory 49 for executingprogramming associated with the encoding. In addition, it will beappreciated that elements of the present invention can be implemented asprogramming stored on a media, which can be accessed for execution by aCPU for the encoder 10 and/or decoder 50.

In the decoder 50 of FIG. 2, decoding blocks 52 are shown along with acomputer processing means 78, which is substantially a subset of theelements contained in the encoder, shown in FIG. 1, operating onreference frames 54 and encoded signal 56 toward outputting a decodedvideo 76. The decoder blocks receive an encoded video signal 56 whichare processed through a CABAC entropy decoder 58, inverse quantization60 is performed using the extended chroma QP determination 62 accordingto an embodiment of the invention. Summing 66 is shown between theinverse transform 64 output and the selection between inter prediction68 shown with motion compensation 70 and intra prediction 72. Outputfrom summing junction 66 is received by filter 74.

It should be appreciated that the decoder can be implemented with aprocessing means 78 which comprises at least one processing device 80(central processing unit (CPU), microcontroller, application specificintegrated circuit (ASIC) containing a computer processor, parallelprocessing devices, or other devices configured for executing programmedinstructions) and at least one memory 82 for executing programmingassociated with the encoding. In addition, it will be noted thatelements of the present invention can be implemented as programmingstored on a media, wherein said media can be accessed for execution byprocessing device (CPU) 80.

It should be appreciated that the programming for the encoder anddecoder is executable from the memory which is a tangible (physical)computer readable media that is non-transitory in that it does notmerely constitute a transitory propagating signal, but is actuallycapable of retaining programming, such as within any desired form andnumber of static or dynamic memory devices. These memory devices neednot be implemented to maintain data under all conditions (e.g., powerfail) to be considered herein as non-transitory media.

As previously mentioned, in the current HEVC test model (HM 5.0), thevalue of chroma QP is derived from luma QP using g_aucChromaScale[52],with chroma QP capable of values in the range of from 0 to 39. However,using that mechanism when luma QP exceeds a value of 48, chroma remainsat a value of 39. In response to this a large portion of the bitrate maybe used for chroma instead of luma.

FIG. 3 depicts a graph of chroma QP mapping from luma QP in the currentHEVC test model (HM 5.0). It can be seen in this graph that chroma QP isforced away from following luma QP for larger QP values. Statisticsindicate, however, that in some cases luma QP and chroma QP remainclose, which is not expected.

FIG. 4 illustrates a general strategy for extending chroma QP in whichthe mapping table g_aucChromaScale[52] is replaced 90, and chroma isextended, such as preferably to have a range which spans that of lumaQP. The example flowchart shown with chroma QP being set equal 92 toluma QP (preferably plus chroma offsets), thus allowing chroma QP toattain a range from 0 to 51.

More particularly, the QP values can be determined for the embodiment ofFIG. 4 according to the following equations:

QP _(Cb)=Clip(0, 51, QP _(Y) +Cb_QP_offset)   (1)

QP _(Cr)=Clip(0, 51, QP _(Y) +Cr_QP_offset)   (2)

First, it should be noted that in video coding in the YCbCr color space,Y represents the luma component, while and Cb and Cr represent twodifferent chroma components for which quantization parameters are needed(QP_(Cb) and QP_(Cr)), although one of ordinary skill in the art willappreciate that the present invention can be readily adapted for use inany desired color space. In eqs. (1) and (2), Cb_QP_offset andCr_QP_offset are the two Chroma QP offset parameters introduced in thedocument JCTVC-G509 for Geneva meeting (MPEG number m22073). It has beenfound that using the extended chroma QPs according to the aboveembodiment provides beneficial performance enhancement, even with thissimplified design.

FIG. 5 illustrates an example embodiment of extending chroma QP at theslice level, taking into account signaling at the slice header utilizingslice_qp_delta_cb and slice_qp_delta_cr. First, luma QP is determined110, then picture level chroma QP offsets are added 112, followed byadding 114 slice level chroma QP offsets, whereafter chroma QP isdetermined 116 with g_aucChromaScale[52].

Equations (3) and (4) below demonstrate this form of slice levelextended chroma QP determination.

QP _(Cb)=Clip(0,51,g_aucChromaScale[QP _(Y)+Cb_QP_offset+slice_qp_delta_cb]  (3)

QP _(Cr)=Clip(0,51,g_aucChromaScale[QP_(Y)+Cr_QP_offset+slice_qp_delta_cr])   (4)

The slice level syntax provides for signaling chroma QP offsets (Cb andCr). The slice level chroma QP extension is particularly well-suited foruse in slice level parallel processing within the coding system.

Embodiments of the present invention may be described with reference toflowchart illustrations of methods and systems according to embodimentsof the invention, and/or algorithms, formulae, or other computationaldepictions, which may also be implemented as computer program products.In this regard, each block or step of a flowchart, and combinations ofblocks (and/or steps) in a flowchart, algorithm, formula, orcomputational depiction can be implemented by various means, such ashardware, firmware, and/or software including one or more computerprogram instructions embodied in computer-readable program code logic.As will be appreciated, any such computer program instructions may beloaded onto a computer, including without limitation a general purposecomputer or special purpose computer, or other programmable processingapparatus to produce a machine, such that the computer programinstructions which execute on the computer or other programmableprocessing apparatus create means for implementing the functionsspecified in the block(s) of the flowchart(s).

Accordingly, blocks of the flowcharts, algorithms, formulae, orcomputational depictions support combinations of means for performingthe specified functions, combinations of steps for performing thespecified functions, and computer program instructions, such as embodiedin computer-readable program code logic means, for performing thespecified functions. It will also be understood that each block of theflowchart illustrations, algorithms, formulae, or computationaldepictions and combinations thereof described herein, can be implementedby special purpose hardware-based computer systems which perform thespecified functions or steps, or combinations of special purposehardware and computer-readable program code logic means.

Furthermore, these computer program instructions, such as embodied incomputer-readable program code logic, may also be stored in acomputer-readable memory that can direct a computer or otherprogrammable processing apparatus to function in a particular manner,such that the instructions stored in the computer-readable memoryproduce an article of manufacture including instruction means whichimplement the function specified in the block(s) of the flowchart(s).The computer program instructions may also be loaded onto a computer orother programmable processing apparatus to cause a series of operationalsteps to be performed on the computer or other programmable processingapparatus to produce a computer-implemented process such that theinstructions which execute on the computer or other programmableprocessing apparatus provide steps for implementing the functionsspecified in the block(s) of the flowchart(s), algorithm(s), formula(e), or computational depiction(s).

From the discussion above it will be appreciated that the invention canbe embodied in various ways, including the following:

1. An apparatus for performing video coding, comprising: a computerprocessor configured for receiving video; and programming executable onsaid computer processor for video coding by performing steps comprising:performing inter-prediction and/or intra-prediction for reducingtemporal and/or spatial redundancies within the received video;determining a luma quantization parameter (QP); adding picture levelchroma QP offsets to said luma QP; determining chroma QP values, basedon the luma QP and picture level chroma QP offsets, said chroma QPvalues having an extended range; performing a transform during encodingof said received video and/or performing inverse transform duringdecoding of said received video; and utilizing said chroma QP values,performing a quantization during encoding of said received video and/orperforming inverse quantization during decoding of said received video.

2. The apparatus of any of the previous embodiments, wherein saidprogramming further performs steps comprising adding slice level chromaQP offsets prior to said determination of the chroma QP values.

3. The apparatus of any of the previous embodiments, wherein saidprogramming further performs steps comprising adding slice level chromaQP offsets for an apparatus utilizing slice level parallel processing.

4. The apparatus of any of the previous embodiments, wherein saidextended chroma QP range is equal to the luma QP range.

5. The apparatus of any of the previous embodiments, wherein saidprogramming determines the chroma QP values by using the picture levelchroma QP offsets for chroma components of a color space with anexisting chroma scale table.

6. The apparatus of any of the previous embodiments, wherein theexisting chroma scale table comprises g_aucChromaScale[52].

7. The apparatus of any of the previous embodiments, wherein saidprogramming determines the chroma QP values by using picture levelchroma offsets and replacing an existing chroma scale table.

8. The apparatus of any of the previous embodiments, wherein said videocoding is performed according to a High Efficiency Video Coding (HEVC)standard.

9. The apparatus of any of the previous embodiments, wherein saidcomputer processor comprises a coder/decoder (CODEC).

10. An apparatus for performing video coding, comprising: a computerprocessor configured for receiving video; and programming executable onsaid computer processor for video coding by performing steps comprising:performing inter-prediction and/or intra-prediction for reducingtemporal and/or spatial redundancies within the received video;determining a luma quantization parameter (QP); adding picture levelchroma QP offsets for Cb and Cr to said luma QP; adding slice levelchroma QP offsets for Cb and Cr to said luma QP; determining chroma QPvalues for Cb and Cr, based on the luma QP, as well as picture level andslice level chroma QP offsets for Cb and Cr, wherein said chroma QPvalues have a QP range equal to the luma QP; performing a transformduring encoding of said received video, and/or inverse transform duringdecoding of said received video; utilizing said chroma QP values for Cband Cr, performing quantization during encoding of said received video,and/or performing inverse quantization during decoding of said receivedvideo.

11. The apparatus of any of the previous embodiments, wherein duringdecoding said received video comprises an encoded video.

12. The apparatus of any of the previous embodiments, wherein saidapparatus is configured for slice level parallel processing.

13. The apparatus of any of the previous embodiments, wherein saidprogramming determines the chroma QP values using the picture level andslice level chroma QP offsets for Cb and Cr with an existing chromascale table.

14. The apparatus of any of the previous embodiments, wherein theexisting chroma scale table comprises g_aucChromaScale[52].

15. The apparatus of any of the previous embodiments, wherein saidprogramming determines chroma QP using luma QP as well as the picturelevel and slice level chroma QP offsets for Cb and Cr, and replacing anexisting chroma scale table.

16. The apparatus of any of the previous embodiments, wherein said videocoding is performed according to a High Efficiency Video Coding (HEVC)standard.

17. The apparatus of any of the previous embodiments, wherein saidcomputer processor comprises a coder/decoder (CODEC).

18. A method of performing video coding, comprising: performinginter-prediction and/or intra-prediction for reducing temporal and/orspatial redundancies of received video within a video encoder and/ordecoder; determining a luma quantization parameter (QP); adding picturelevel chroma QP offsets for Cb and Cr; adding slice level chroma QPoffsets for Cb and Cr; determining extended chroma QP values for Cb andCr based on the luma QP and picture level and slice level chroma QPoffsets for Cb and Cr; and performing a transform during encoding ofsaid received video and/or performing an inverse transform duringdecoding of said received video; and utilizing said chroma QP values forCb and Cr, performing quantization during encoding of said receivedvideo, and/or performing inverse quantization during decoding of saidreceived video.

19. The method of any of the previous embodiments, wherein said chromaQP vales have a range equal to the luma QP range.

20. The method of any of the previous embodiments, wherein saiddetermining of the chroma QP is performed using the picture level andslice level chroma QP offsets for Cb and Cr, either with an existingchroma scale table, or by replacing the existing chroma scale table.

Although the description above contains many details, these should notbe construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. Therefore, it will be appreciated that the scope ofthe present invention fully encompasses other embodiments which maybecome obvious to those skilled in the art, and that the scope of thepresent invention is accordingly to be limited by nothing other than theappended claims, in which reference to an element in the singular is notintended to mean one and only one” unless explicitly so stated, butrather one or more.” All structural and functional equivalents to theelements of the above-described preferred embodiment that are known tothose of ordinary skill in the art are expressly incorporated herein byreference and are intended to be encompassed by the present claims.Moreover, it is not necessary for a device or method to address each andevery problem sought to be solved by the present invention, for it to beencompassed by the present claims. Furthermore, no element, component,or method step in the present disclosure is intended to be dedicated tothe public regardless of whether the element, component, or method stepis explicitly recited in the claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. 112, sixth paragraph, unlessthe element is expressly recited using the phrase “means for.”

TABLE 1 Specification of QP_(C) as a function of QP_(Y) QP_(Y) <30 30 3132 33 34 35 36 37 38 39 QP_(C) =QP_(Y) 29 30 31 32 32 33 34 34 35 35QP_(Y) 40 41 42 43 44 45 46 47 48 49 50 51 QP_(C) 36 36 37 37 37 38 3838 39 39 39 39

What is claimed is:
 1. A decoding apparatus, comprising: a decoding unitconfigured to: decode a bit stream that includes a picture level chromaquantization parameter (QP) offset and a slice level chroma QP offset togenerate quantization data; and obtain the picture level chroma QPoffset and the slice level chroma QP offset from the bit stream; asetting unit configured to set a chroma QP by use of a parameter withthe picture level chroma QP offset and the slice level chroma QP offsetobtained by the decoding unit added to a luma QP; and an inversequantization unit configured for inverse quantization of quantizationdata generated by the decoding unit, by use of the chroma QP set by thesetting unit.
 2. The decoding apparatus of claim 1, wherein the slicelevel chroma QP is included in a slice header of the bit stream, and thedecoding unit is further configured to obtain a slice level chroma QPoffset from the slice header of the bit stream.
 3. The decodingapparatus of claim 2, wherein the picture level chroma QP is included ina picture parameter set of the bit stream, and the decoding unit isfurther configured to obtain a picture level chroma QP offset from thepicture parameter set of the bit stream.
 4. The decoding apparatus ofclaim 3, wherein the setting unit is further configured to set thechroma QP, by use of a mapping table that maps a chroma QP to aparameter with a picture level chroma QP offset and a slice level chromaQP offset added to a luma QP.
 5. The decoding apparatus of claim 4,further comprising an inverse orthogonal transforming unit configuredfor inverse orthogonal-transformation of transform data generated by theinverse quantization unit.
 6. A decoding method, comprising: decoding abit stream that includes a picture level chroma quantization parameter(QP) offset and a slice level chroma QP offset to generate quantizationdata; obtaining a picture level chroma QP offset and a slice levelchroma QP offset from the bit stream; setting a chroma QP by use of aparameter with the picture level chroma QP offset and the slice levelchroma QP offset added to a luma QP; and inverse quantizing thegenerated quantization data by use of the chroma QP.
 7. The decodingmethod of claim 6, wherein the slice level chroma QP is included in aslice header of the bit stream, and the obtaining step includesobtaining a slice level chroma QP offset from a slice header of the bitstream.
 8. The decoding method of claim 6, wherein the picture levelchroma QP is included in a picture parameter set of the bit stream, andthe obtaining step further includes obtaining a picture level chroma QPoffset from a picture parameter set of the bit stream.
 9. The decodingmethod of claim 6, wherein the setting step further includes setting thechroma QP, by use of a mapping table that maps a chroma QP to aparameter with a picture level chroma QP offset and a slice level chromaQP offset added to a luma QP.
 10. The decoding method of claim 6,further comprising inverse orthogonal-transforming transform datagenerated by the inverse quantizing.